Various laser ignition devices have hitherto been developed. For example, a laser ignition device is known in which the pulse time width of laser applied and focused in a cylinder is set to be a pulse time width which realizes a minimum ignition input energy (for example, see JP2006144726 (A)).
Meanwhile, a laser ignition system provided with a pulse laser oscillator and a lens which focuses a pulse laser beam in a cylinder is configured such that a pulse laser beam having a normalized fluence volume greater than 0.1 is generated (for example, see JP2016-507903 (A)).
In the conventional laser ignition device described above, the pulse time width of laser is not optimized. Thus, in the conventional laser ignition device, the laser pulse energy necessary for ignition is large and the ignition energy efficiency is low.
The present invention has been made in view of the above-described problem. An object of the present invention is to provide a laser ignition device in which the ignition efficiency is improved and the laser pulse energy necessary for ignition is minimized by optimizing the pulse time width of laser.
A laser ignition device of the present invention for solving the problem includes: a pulse laser oscillator configured to output a laser beam having a wavelength λ [μm] and a beam quality M2; an energy controller configured to control energy of pulse laser outputted from the pulse laser oscillator; a lens having a focal length f [mm] and configured to focus the pulse laser outputted from the pulse laser oscillator; and a pulse time width controller configured to control a pulse time width of the pulse laser, wherein the pulse time width controller controls the pulse time width of the pulse laser to be 0.6 to 2 ns. Accordingly, the dielectric breakdown intensity takes constant low values.
Another laser ignition device of the present invention for solving the problem includes: a pulse laser oscillator configured to output a laser beam having a wavelength λ [μm] and a beam quality M2; an energy controller configured to control energy of pulse laser outputted from the pulse laser oscillator; a lens having a focal length f [mm] and configured to focus the pulse laser outputted from the pulse laser oscillator; and a pulse time width controller configured to control a pulse time width of the pulse laser, wherein the pulse time width controller controls the pulse time width of the pulse laser to be 0.57 to 0.63 ns. Accordingly, the dielectric breakdown threshold fluence is minimized.
In the laser ignition device described above, preferably, the M2 is less than 4. Accordingly, the diameter of the light-focused spot is decreased, and the laser pulse energy necessary for ignition is minimized.
In the laser ignition device described above, when a beam diameter of the pulse laser incident on the lens is D [mm], fλ/D is set to be 1.4 to 3.5 μm, preferably, 2.1 to 2.8 μm.
The diameter of the light-focused spot where light is focused by a lens is proportional to fλ/D. Therefore, when f is decreased, the diameter of the light-focused spot is decreased, and thus, laser pulse energy necessary for dielectric breakdown is minimized. However, when f is decreased too much, electrons generated through dielectric breakdown diffuse to the outside of the light-focused spot, and thus, loss is increased. On the other hand, when f is increased, the diameter of the light-focused spot is increased, and thus, laser pulse energy necessary for dielectric breakdown is increased. That is, a trade-off relationship exists between low (high) dielectric breakdown energy and high (low) diffusion loss. An experiment by the inventors revealed that when fλ/D is set to be 1.4 to 3.5 μm, preferably, 2.1 to 2.8 μm, dielectric breakdown energy is minimized.
The dielectric breakdown intensity is caused to take constant low values, and the ignition energy efficiency is enhanced.